Recently, with a finer LCD (Liquid Crystal Display) of a mobile telephone, there is a demand for enhancement of a data transmission speed between the LCD and an application processor. In the data transmission wiring between circuit boards, there is also a demand for reduction of the number of wirings or a high bending property as a low profile of the mobile telephone progresses. Particularly, in a sliding type mobile telephone including a sliding mechanism that moves two circuit boards in parallel, it is necessary to generate wiring bending in a space (a distance between circuit boards is less than 3 mm) formed between the circuit boards. Therefore, in the sliding type mobile telephone, there is the noticeable demand for the high bending property of the wiring.
Conventionally, a flexible printed circuit (FPC) board is used to connect the circuit boards in the sliding type mobile telephone. For example, Patent Documents 1 and 2 disclose techniques of connecting the circuit boards using the flexible printed circuit board.
Patent Document 1 discloses a flexible printed circuit board whose outside resin film has an elastic modulus of 6 GP or more. The flexible printed circuit board is provided in an outside insulating layer that becomes an outside during bending. Therefore, a bending-resistant property of the flexible printed circuit board is improved to prevent disconnection of a conductive circuit constituting a conductive circuit layer, even if the flexible printed circuit board is repeatedly bent in one direction. In the technique disclosed in Patent Document 2, a thickness, the elastic modulus, a sliding radius and the like are defined for each of a plurality of layers (such as an electrically insulating layer, bonding agent layer, and a conductive layer) constituting the flexible printed circuit board, thereby improving the bending-resistant property of the flexible printed circuit board. In the techniques disclosed in Patent Documents 1 and 2, repeated bending durability is improved by controlling the elastic modulus and thickness of the resin material layer in the flexible printed circuit board.
Separately of the connection between the circuit boards using the flexible printed board, there has been tried a method for connecting the LCD and the application processor using an optical transmission path such as an optical waveguide to transmit a data signal as an optical signal.
The optical waveguide has a double structure in which a center called a core is coated with a casing called a clad, and a refractive index of the core is higher than that of the clad. Therefore, the optical signal incident to the core is propagated by repeating total reflection in the core.
For example, using an optical transmission module, the high-speed, large-capacity data signal can be transmitted to an application circuit board from a main control board mounted in the mobile telephone. The optical transmission module is excellent as a data transmission module.
For example, Patent Document 3 discloses a technique of applying the optical transmission module to the mobile telephone. In the optical transmission module disclosed in Patent Document 3, a plurality of mirrors that deflect light at substantially 90° are disposed between a light emitting element and a light receiving element, and a plurality of optical transmission paths are provided between the light emitting element and the mirror, between the plurality of mirrors, and between the light receiving element and the mirror. With this configuration, the optical transmission module can be rotated about a plurality of rotating axes, and bending is not generated in the optical transmission path. Therefore, an optical loss caused by the bending of the optical transmission path can be avoided to realize the mobile telephone that can deal with various operations such as the rotation.    Patent Document 1: Japanese Unexamined Patent Publication No. 2006-310643 (published on Nov. 9, 2006)    Patent Document 2: Japanese Unexamined Patent Publication No. 2009-194179 (published on Aug. 27, 2009)    Patent Document 3: Japanese Unexamined Patent Publication No. 2005-252334 (published on Sep. 15, 2005)